The present invention relates to analog-to-digital conversion, and more particularly to analog-to-digital conversion of a signal at radio frequency sampling rates.
The role of telecommunications continues to grow in today""s world. This is true not only in business settings (where communications are very often vital), but also in the day-to-day lives of individuals. With the advent of mobile communications (e.g., cellular telephone systems), individuals with hectic lifestyles find that they are more and more dependent on their personal communication devices to keep in touch with business associates as well as with friends and family.
Because of this increased dependence, the drive to make mobile communication devices more flexible and more reliable grows steadily. Research continues in an effort to reduce the size and power consumption of portable communications devices, in order to make them more convenient to carry while increasing their useful life between recharging.
In an effort to accomplish these goals, the trend has been to substitute digital technology for analog technology. In addition to achieving the goals of reduced size and power consumption, the substitution of digital technology for analog technology has resulted in increased quality of service because analog components are very often responsible for introducing problems like nonlinearities, distortions and spurious reception. Spurious reception is caused by mixing a higher harmonic of a spurious signal with a higher harmonic of the local oscillator, thereby generating a signal close to the intermediate frequency, fIF:
fIF≈|m xc2x7fSPxe2x88x92nxc2x7fLO|
where fSP is the spurious signal""s frequency, fLO is the local oscillator frequency, and m and n are the order of the signal and the local oscillator harmonic, respectively. Solving for fSP gives:       f    SP    ≈                    ±                  1          m                    ⁢              f        IF              +                  n        m            ·              f        LO            
where the positive sign holds if the local oscillator frequency is above that of the wanted signal, and the negative sign occurs if it is below that of the wanted signal.
Despite the desire to utilize digital technology as much as possible, state of the art receivers continue to include one or two analog Intermediate Frequency (IF) stages before the signal is sampled by a multi-bit analog-to-digital (A/D) converter. The reason for this is that radio frequencies intended to be used for mobile radio applications are in the range of 1 or 2 gigahertz or above. Because conventional multi-bit A/D-converters are characterized by a limited input bandwidth, sampling at the radio frequency (RF) rate has not been possible. And, without A/D conversion, analog technology is the only means available for initially processing the received RF signal.
S. Yang, et. al., xe2x80x9cA tunable bandpass sigma-delta A/D conversion for mobile communication receiver,xe2x80x9d 1994 IEEE 44th Vehicular Technology Conference, pp. 1346-1350, vol. 2 (1994) describes a receiver with one analog downconversion and techniques for tuning such a receiver by means of sigma-delta modulation.
It is therefore an object of the present invention to provide improved methods and apparatuses for reception of radio frequency signals.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved in a radio receiver that receives a radio frequency signal, and uses sigma-delta analog-to-digital conversion techniques that sample the radio frequency signal at a sampling rate and generates therefrom 1-bit digital samples representing an intermediate frequency signal. Whether or not the sampling rate is at, above or below the Nyquist rate of the radio frequency signal, the sampling rate is preferably many times higher than the signal bandwidth. Having generated the intermediate frequency signal, demodulation is then used to generate in-phase and quadrature samples from the intermediate frequency signal. An advantage of this technique is that demodulation may be performed in a purely digital manner.
In another aspect of the invention, the intermediate frequency is a difference between the radio frequency and a closest harmonic of the sampling rate.
In some embodiments, demodulation comprises generating a first mixed signal by combining the 1-bit digital samples representing the intermediate frequency signal with a first sequence representing a cosine mixing signal; and generating a second mixed signal by combining the 1-bit digital samples representing the intermediate frequency signal with a second sequence representing a sine mixing signal. The first and second mixed signals are then decimated to generate the in-phase and quadrature samples. In these embodiments, the intermediate frequency may be one fourth of the sampling rate.
In yet another aspect of the invention, exclusive-OR logic gates may be used to generate the first and second mixed signals.
In still another aspect of the invention, demodulation may alternatively be performed by receiving the 1-bit digital samples representing the intermediate frequency signal and generating therefrom first and second decimated signals, wherein: the first decimated signal is based on the 1-bit digital samples; the second decimated signal is based on a time-shifted version of the 1-bit digital samples; each of the first and second decimated signals has one sample for every number, N, of 1-bit digital samples representing the intermediate frequency; and the time-shifted version of the 1-bit digital samples is the 1-bit digital samples delayed by an amount, xcex94n cycles of the sampling rate. xcex94n may represent an odd multiple of a quarter period of the intermediate frequency. The first and second decimated signals are then bandpass filtered to generate the respective in-phase and quadrature samples.
In yet another aspect of the invention, demodulation may alternatively be performed by considering the intermediate frequency to be a first intermediate frequency, and bandpass filtering and decimating the intermediate frequency signal to generate a digital signal having a second intermediate frequency. A demodulator is then used for reconstructing the in-phase and quadrature samples from the digital signal having the second intermediate frequency. In these embodiments, the bandpass filtering as a bandpass characteristic around the first intermediate frequency.